<!DOCTYPE HTML>
<html>

<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>计时器</title>

  <script type="text/javascript">
    var num = 0;
    var i;

    function startCount() {
      document.getElementById('count').value = num;
      num = num + 1;
      i = setTimeout("startCount()", 1000);
    }

    function stopCount() {
      clearTimeout(i)
    }
  </script>
</head>

<body>
  <form>
    <input type="text" id="count" />
    <input type="button" value="Start" onclick="startCount()" />
    <input type="button" value="Stop" onclick="stopCount()" />
  </form>
</body>

</html>